This invention relates to non-carbon metal-based anodes for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, and to methods for their fabrication and reconditioning, as well as to electrowinning cells containing such anodes and their use to produce aluminium.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still not less than 950xc2x0 C. in order to have a sufficiently high solubility and rate of dissolution of alumina and high electrical conductivity of the bath.
The anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO2 and small amounts of CO and fluoride-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than ⅓ higher than the theoretical amount of 333 Kg/Ton
The frequent substitution of the anodes in the cells is still a clumsy and unpleasant operation. This cannot be avoided or greatly improved due to the size and weight of the anode and the high temperature of operation.
Several improvements were made in order to increase the lifetime of the anodes of aluminium electrowinning cells, usually by improving their resistance to chemical attacks by the cell environment and air to those parts of the anodes which remain outside the bath. However, most attempts to increase the chemical resistance of anodes were coupled with a degradation of their electrical conductivity.
U.S. Pat. No.4,614,569 (Duruz/Derivaz/Debely/Adorian) describes non-carbon anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained by the addition of cerium compounds to the molten cryolite electrolyte. This made it possible to have a protection of the surface only from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidized copper-nickel surface on an alloy substrate with a protective barrier layer. However, full protection of the alloy substrate was difficult to achieve.
A significant improvement was described in U.S. Pat. No. 5,510,008, and in International Application WO96/12833 (Sekhar/Liu/Duruz) involved a anode having a micropyretically produced body from a combination of nickel, aluminium, iron and copper and oxidising the surface before use or in-situ during electrolysis. By said micropyretic methods materials have been obtained whose surfaces when oxidised are active for the anodic reaction and whose metallic interior has low electrical resistivity to carry a current from high electrical resistant surface to the busbars. However it would be useful, if it were possible, to simplify the manufacturing process of these materials obtained from powders and increase their life to make their use economic.
Metal or metal based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. Many attempts were made to use metal-based anodes for aluminium production, however they were never adopted by the aluminium industry because of their poor performance.
An object of the invention is to substantially reduce the consumption of the active anode surface of an aluminium electrowinning anode which is attacked by the nascent oxygen by enhancing the reaction of nascent oxygen to biatomic molecular gaseous oxygen.
Another object of the invention is to provide a coating for an aluminium electrowinning anode which has a high electrochemical activity and also a long life and which can be replaced as soon as such activity decreases or when the coating is worn out.
A major object of the invention is to provide an aluminium electrowinning anode which has no carbon so as to eliminate carbon-generated pollution and reduce the cost of operation.
The invention provides a non-carbon metal-based anode of a cell for the electrowinning of aluminium, in particular by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte. The anode comprises an electrically conductive metal substrate resistant to high temperature, the surface of which becomes passive and substantially inert to the electrolyte, and an electrochemically active coating adherent to the surface of the metal substrate making and keeping the surface of the anode conductive and electrochemically active for the oxidation of oxygen ions present at the electrolyte interface.
Whereas conventional coatings are usually used to protect a conductive substrate of a cell component from chemical and/or mechanical attacks destroying the substrate, this particular treatment is applied in the form of a coating onto a passivatable substrate to maintain the anode surface conductive and electrochemically active and protect it from electrolyte attack wherever the coating covers the surface even though the coating may be imperfect or incomplete.
This allows the coated surfaces of the anode to remain electrochemically active during electrolysis, while the remaining parts of the surface of the metal substrate become inert to the electrolyte. This passivation property offers a self-healing effect, i.e. when the surface of the anode is imperfectly covered, damaged or partly worn out, parts of the metal substrate which come into contact with the electrolyte are automatically passivated during electrolysis and become inert to the electrolyte and not corroded.
Metal substrates providing for this self-healing effect in molten fluoride-based electrolyte may be made of one or more metals selected from nickel, cobalt, chromium, molybdenum, tantalum and the Lanthanide series of the Periodic Table, and their alloys or intermetallics, such as nickel-plated copper.
The coatings usually comprise:
a) at least one electrically conductive and electrochemically active constituent,
b) an electrocatalyst, and
c) a bonding material substantially resistant to cryolite and oxygen for bonding these constituents together and onto the passivatable metal substrate.
These constituents are usually co-applied though it is possible to provide sequential application of the different constituents.
The presence of one or more electrocatalysts is desirable, although not essential for the invention. Likewise the presence of bonding material is not always necessary.
Coatings can be obtained by applying their active constituents and their precursors by various methods which can be different for each constituent and can be repeated in several layers. For example, a coating can be obtained by directly applying a powder onto the passivatable metal substrate or constituents of the coating may be applied from a slurry or suspension containing colloidal or polymeric material. The colloidal material can be a binder solely or can be part of the active material. The colloidal material may include at least one colloid selected from colloidal alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia, tin oxide, zinc oxide and colloid containing the active material.
When a slurry or a suspension containing colloidal material is applied the dry colloid content corresponds to up to 50 weight % of the colloid plus liquid carrier, usually from 10 to 20 weight %.
The coating can be applied on the substrate by plasma spraying, physical vapor deposition (PVD), chemical vapor deposition (CVD), electrodeposition or callendering rollers. A slurry or a dispersion is preferably applied by rollers, brush or spraying.
Usually the electrochemically active constituent(s) is/are selected from oxides, oxyfluorides, phosphides, carbides and combinations thereof.
The oxide may be present in the electrochemically active layer as such, or in a multi-compound mixed oxide and/or in a solid solution of oxides. The oxide may be in the form of a simple, double and/or multiple oxide, and/or in the form of a stoichiometric or non-stoichiometric oxide.
The oxides may be in the form of spinels and/or perovskites, in particular spinels which are doped, non-stoichiometric and/or partially substituted. Doped spinels may comprise dopants selected from Ti4+, Zr4+, Sn4+, Fe4+, Hf4+, Mn4+, Fe3+, Ni3+, Co3+, Mn3+, Al3+, Cr3xe2x80x2, Fe2+, Ni2+, Co2+, Mg2+, Mn2+, Cu2+, Zn2+ and Li+.
Such a spinel may be a ferrite, in particular a ferrite selected from cobalt, manganese, molybdenum, nickel and zinc, and mixtures thereof. The ferrite may be doped with at least one oxide selected from the group consisting of chromium, titanium, tantalum, tin, zinc and zirconium oxide.
Nickel-ferrite or nickel-ferrite based constituents are advantageously used for their resistance to electrolyte and may be present as such or partially substituted with Fe2+.
The coating may also contain a chromite which is usually selected from iron, cobalt, copper, manganese, beryllium, calcium, strontium, barium, magnesium, nickel and zinc chromite.
The electrochemically active constituents of the coating may be selected from iron, chromium, copper and nickel, and oxides, mixtures and compounds thereof, as well as a Lanthanide as an oxide or an oxyfluoride such as cerium oxyfluoride, and mixtures thereof.
When an electrocatalyst is present in the coating it is selected preferably from noble metals such as iridium, palladium, platinum, rhodium, ruthenium, or silicon, tin and zinc, the Lanthanide series of the Periodic Table and mischmetal oxides, and mixtures and compounds thereof.
Coatings can be formed with or without reaction at low or high temperature. A reaction can either take place among the constituents of the coating; or between the constituents of the coating and the passivatable metal substrate. When no reaction takes place to form the coating the active constituents must already be present in the applied material, for example in a slurry or suspension applied onto the substrate.
In order to manufacture these anodes any electrically conductive and heat-resisting materials may be used. However, metals which do not offer the self-healing effect can only be used as metal cores which must be coated with a layer forming the passivatable metal substrate having this self-healing effect particularly when exposed to a fluoride-containing electrolyte, such as cryolite.
The metal core may comprise metals, alloys, intermetallics, cermets and conductive ceramics, such as metals selected from copper, chromium, cobalt, iron, aluminium, hafnium, molybdenum, nickel, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and combinations and compounds thereof.
For instance, the core may be made of an alloy comprising 10 to 30 weight % of chromium, 55 to 90 weight % of at least one of nickel, cobalt and/or iron and 0 to 15 weight % of at least one of aluminium, hafnium, molybdenum, niobium, silicon, tantalum, tungsten, vanadium, yttrium and zirconium.
The core may be covered with an oxygen barrier layer. This layer may be obtained by oxidising the surface of the core when it contains chromium and/or nickel or by applying a precursor of the oxygen barrier layer onto the core and heat treating. Usually, the oxygen barrier layer comprises chromium oxide and/or black non-stoichiometric nickel oxide.
The oxygen barrier layer may be covered in turn with at least one protective layer consisting of copper or copper and at least one of nickel and cobalt, and/or (an) oxide(s) thereof to protect the oxygen barrier layer by inhibiting its dissolution into the electrolyte. For instance, the oxygen barrier layer may be coated first with a nickel layer and then with a copper layer, heat treated for several hours in an inert atmosphere, such as 5 hours at 1000xc2x0 C. in argon, to interdiffuse the nickel and the copper layer, and upon heat treatment in an oxidising media, such as an air oxidation for 24 hours at 1000xc2x0 C., the interdiffused and oxidised nickel-copper layer constitutes a good a protective layer.
The invention relates also to a method of manufacturing the described non-carbon metal-based anode. The method comprises coating a substrate of electrically conductive metal resistant to high temperature the surface of which during electrolysis becomes passive and substantially inert to the electrolyte with at least one layer containing electrochemically active constituents or precursors thereof and heat-treating the or each layer on the substrate to obtain a coating adherent to the metal substrate making the surface of the anode electrochemically active for the oxidation of oxygen ions present at the electrolyte interface.
The method of the invention can be applied for reconditioning the non-carbon metal-based anode when at least part of the active coating has been dissolved or rendered non-active or dissolved. The method comprises clearing the surface of the substrate before re-coating said surface with a coating adherent to the passivatable metal substrate once again making the surface of the anode electrochemically active for the oxidation of oxygen ions.
Another aspect of the invention is a cell for the production of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte, in particular a fluoride-based electrolyte or a cryolite-based electrolyte or cryolite, having non-carbon metal-based anodes comprising an electrically conductive passivatable metal substrate and a conductive coating having an electrochemically active surface as described hereabove.
Preferably, the cell comprises at least one aluminium-wettable cathode. Even more preferably, the cell is in a drained configuration by having at least one drained cathode on which aluminium is produced and from which aluminium continuously drains.
The cell may be of monopolar, multi-monopolar or bipolar configuration. A bipolar cell may comprise the anodes as described above as a terminal anode or as the anode part of a bipolar electrode.
Preferably, the cell comprises means to improve the circulation of the electrolyte between the anodes and facing cathodes and/or means to facilitate dissolution of alumina in the electrolyte. Such means can for instance be provided by the geometry of the cell as described in co-pending application PCT/IB98/00161 (de Nora/Duruz) or by periodically moving the anodes as described in co-pending application PCT/IB98/00162 (Duruz/Bellò).
The cell may be operated with the electrolyte at conventional temperatures, such as 950 to 970xc2x0 C., or at reduced temperatures as low as 750xc2x0 C.
The invention also relates to the use of such an anode for the production of aluminium in a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte, wherein oxygen ions in the electrolyte are oxidised and released as molecular oxygen by the electrochemically active anode coating.
The invention will now be described in the following examples.